This invention relates to capacitors and is directed more specifically to improvements in a capacitor of the type having inner and outer electrodes formed on a tubular ceramic body. The invention is also specifically directed to a process for the manufacture of such an improved ceramic capacitor.
A capacitor having inner and outer electrodes formed on a tubular ceramic body of barium titanate or the like (FIG. 1) has been known and used extensively. Each electrode of this type of capacitor usually takes the form of a silver layer or the lamination of nickel, copper and silver layers. Formed by firing a coating of pasted silver on the ceramic body, the silver electrodes have the advantage of affording a strong joint with the dielectric and of permitting the ready soldering thereto of leads or other means for electrical connection, besides being favorable in electrical characteristics. Capacitors with silver electrodes, however, are not mass-producible and are expensive.
The laminated nickel-copper-silver electrodes, on the other hand, are produced by first forming the nickel layer on the ceramic body by the electroless plating method and then by successively electroplating the copper and silver layers on the nickel layer. The silver layer in this latter case can be of minimum thickness because it is intended merely to improve the solderability of the electrodes, so that the cost of the capacitor can be materially reduced. As an additional advantage, this type of capacitor lends itself to mass-production since its electrodes are formed by plating.
The electrodes of this second type have a problem, however, arising particularly when their outermost silver layer is unduly reduced in thickness to economize the precious metal. Such a thin silver layer when heated permits exudation, through its pinholes, of the copper forming the intermediate layer. The copper exudate on oxidation renders the silver layer significantly less solderable, so that leads or other electrical connection means cannot be readily soldered to the electrodes. The poor solderability of the electrodes also results in the formation of poor joints between them and the electrical connection means and, possibly, in the cracking of the tubular ceramic body through exposure to localized heat for a prolonged period of time.